Inorg. Chem. 1984, 23, 3057-3062
3057
Contribution from the Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38
Kinetics of Oxygen Atom Transfer Reactions Involving Oxomolybdenum Complexes. General Treatment for Reactions with Intermediate Oxo-Bridged Molybdenum(V) Dimer Formation MARTHA S. REYNOLDS,la JEREMY M. BERG,Ib and R. H. HOLM*
Received November 2, 1983 A general treatment of the kinetics of the irreversible forward and reverse oxygen-atom transfer reactions MoV'02L2+ X
.& k-l
MoIVOL2+ XO
+
in the presence of the reversible dimerization equilibrium MoV203L4 + Mo0,L2 MoOL2has been developed. The treatment has as its only assumption equilibrium of the dimerization reaction at all points along the reaction coordinate and allows determination of rate constants by spectrophotometry regardless of the relative initial concentrations of the reactants. Previous kinetics analyses were inadequate in scope or accuracy. The method has been applied to reactions in 1,2-dichloroethane at 25 OC. For the systems M o O , ( S ~ C N E ~ ~ ) ~ /the P R rate , constants k , (M-I s-') = 0.071 (PPh,), 0.23 (PPh,Et), 0.43 (PPhEt2), and 0.53 (PEt,) for oxygen atom transfer to substrate. These values monotonically increase with increasing phosphine nucleophilicity. For the system M o O ( S ~ C N E ~ ~ ) ~ / M ~k-, ,S= O ,1.6 X IO4 M-' s- I . The corresponding reaction rate with N-methylmorpholine N-oxide is orders of magnitude larger, suggesting that breaking of the bond to oxygen is the dominant factor in determining reaction rate. The excellent agreement between calculated and observed absorbance vs. time behavior and the previously determined dimerization rate constant of 1470 M-' s-l for L = S,CNEt, complexes justify the assumption in the kinetics analysis. This study provides the most detailed treatment of oxygen atom transfer reactions involving oxomolybdenum(IV,VI) complexes. Prior observations pertinent to these reactions are summarized.
Introduction The significance of oxygen atom transfer reactions effected by transition-element compounds derives in large measure from catalysis of the forward and reverse reactions (1) by molybdenum hydr~xylases.~-~ Prominent examples of enzymes that
x + [O]@ xo
(1)
catalyze the oxidation (X) or reduction (XO) of substrates in the two-electron formal processes X HzO + XO + 2H' 2e- include xanthine oxidase/dehydrogenase, sulfite oxidase, and nitrate reductase. The generalized reaction (2) and its
+
+
+ X Z= MoOLZ + XO M0OL2 + Mo02L K3 = k3/k-3
M002L2
Mo2O3L4
k3
k-3
k2
(2)
observed by Barral et al." in 1972 in their development of a system (L = RzNCSz-)for the catalytic aerial oxidation of triphenylphosphine. In subsequent work, this catalytic aspect has been pursued,l2 the reaction has been used for synthesis of MOOcomple~es,'~-'~ and kineti~s"J~-'~ and thermochemicalZ0properties have been examined. The reverse reaction (2) has not been extensively investigated, and there are few documented examples of reduction of substrates X0.133'932' A complicating feature of nearly all MoO,Lz/X systems is the occurrence of reaction 3, in which MoV-0-MoV complexes are formed. Unlike reaction 2, this process is reversible for some systems and has been detected by electrochemical,23 s p e c t r o p h o t ~ m e t r i c , ~ ~and - ~ ~infrared30 ~ ~ ~ ~ ~ ~ ~methods. ~ " ~ ~ Rate
(3)
component MoVI02and MotVOspecies have frequently been considered as models of enzymic reactions and catalytic respectively. EXAFS analysis of the oxidized form of sulfite oxidase and the reduced forms of this enzyme and xanthine dehydrogenase indicate the presence of the Mooz and MOO groups, respectively.8.9 The forward reaction (2) was first National Science Foundation Predoctoral Fellow: (a) . . 1981-1984: (b) .. 1980-1983. Bray, R. C. In 'The Enzymes", 3rd ed.; Boyer, P. D., Ed.; Academic Press: New York, 1975; Vol. XII,Part B, Chapter 6. Bray, R. C. Adv. Enzymol. 1980, 51, 107. Coughlan, M. P., Ed.; "Molybdenum and Molybdenum-Containing Enzymes"; Pergamon Press: New York, 1980. Newton, W. E.; Otsuka, S., Eds. 'Molybdenum Chemistry of Biological Significance"; Plenum Press: New York, 1980. Stiefel, E. I. Prog. Znorg. Chem. 1977, 22, 1. Reference 4, Chapter 2. Reference 5, pp 279-294. Spence, J. T. Coord. Chem. Rev. 1983,48, 59. Reference 4, Chapter 3. Spence, J. T.; Minelli, M.; Rice, C. A.; Chasteen, N. C.; Scullane, M. In ref 5 , pp 263-278. (a) Cramer, S.P.; Gray, H. B.; Rajagopalan, K. V. J. Am. Chem. Soc. 1979,101,2772. (b) Cramer, S. P.; Wahl, R.; Rajagopalan, K. V. Zbid. 1981, 103,7721. This group is also present in the cyanolyzed (inactive) form of xanthine oxidase/dehydrogenase.*Jo The native form contains the MoV*OS unit.*JO Bordas, J.; Bray, R. C.; Garner, C. D.; Gutteridge, S.; Hasnain, S. S. Biochem. J . 1980, 191, 499.
(1 1) Barral, R.; Bocard, C.; SCrte de Roch, I.; Sajus, L. Tetruhedron Lett. 1972, 1693. (12) (a) Speier, G. Znorg. Chim. Acta 1979, 32, 139. (b) Deli, G.; Speier, G. Transition Met. Chem. (Weinheim, Ger.) 1981, 6, 227. (13) Chen, G. J.-J.; McDonald, J. w.; Newton, w . E. znorg. Chem. 1976, 15, 2612. (14) Newton, w . E.;McDonald, J. w. J . Less-Common Met. 1977,54,51. (1 5) Hyde, J.; Venkatasubramian, K.; Zubieta, J. Znorg. Chem. 1978, 17, 414. (16) Boyd, I. W.; Spence, J. T. Znora. Chem. 1982, 21, 1602. (17) McDonald, D.-B.; Shulman, J. i. Anal. Chem. 1975,47, 2023. (18) Durant, R.; Garner, C. D.; Hyde, M.R.; Mabbs, F. E. J . Chem. SOL., Dalton Trans. 1977, 955. (19) Durant, R.; Garner, C. D.; Hyde, M. R.; Mabbs, F. E.; Parsons, J. R.; Richens, D. J. Less-Common Met. 1977, 54, 459. (20) Watt, G. D.; McDonald, J. W.; Newton, W. E. Chem. Uses Molybdenum, Proc. Znt. ConJ 1976, 2, 216-219; J. Less-Common Mer. 1977, 54, 415. (21) Mitchell, P. C. H.; Scarle, R. J. Chem. Soc., Dalton Trans. 1975, 2552. (22) Newton, W.E.; Watt, G. D.; McDonald, J. W. Chem. Uses Molybdenum, Proc. Znt. ConJ 1979, 3, 259-263. (23) DeHayes, L. J.; Faulkner, H. C.; Doub, W. H., Jr.; Sawyer, D. T. Znorg. Chem. 1975, 14, 2110. (24) Newton, W. E.; Corbin, J. L.; Bravard, D. C.; Searles, J. E.; McDonald, J. W. Znorg. Chem. 1974, 13, 1100. (25) Chen, G. J.-J.; McDonald, J. W.; Newton, W. E. Znorg. Nucl. Chem. Lett. 1976, 12, 697. (26) Miyake, S.; Tanaka, K.; Tanaka, T. J. Chem. SOC.,Dalton Trans. 1981, 292. (27) Matsuda, T.; Tanaka, K.; Tanaka, T. Znorg. Chem. 1979, 18, 454.
0020-166918411323-3057$01.50/00 1984 American Chemical Society
3058 inorganic Chemistry, Vol. 23, No. 20, 1984
and equilibrium constants' 1,17,22,2629 and thermochemical datam have been reported for a number of reactions 3. The majority of these systems involve dithiocarbamate complexes. A proper understanding of reactions 2 and 3 is pertinent to the interpretation of enzymic oxygen atom transfer reactions and to the development of active-site analogues with requisite reactivity. Despite the extensive and contributory investigations of these reactions, existing kinetics treatments are not entirely satisfactory in scope or accuracy. Here we report a more complete kinetics analysis of the coupled reactions (2) and (3), rate constants for the forward reaction (2) with a series of tertiary phosphine substrates, and two demonstrations of the occurrence of the reverse reaction (2). In all cases complexes are derived from the prototypic ligand N,N-diethyldithiocarbamate. Experimental Section Compounds and Solvents. Dioxobis(N,N-diethyldithiocarbamato)molybdenum(VI), Mo02(S2CNEt2)2,was prepared by the method of Moore and Larsen31 and was recrystallized from benzene-hexane = 18 M-I cm-I). Oxobis(N,N-diethyldithiocarbamato)molybdenum(IV), M O O ( S ~ C N E ~was ~ ) ~prepared , as described by Chen et al." and was similarly recrystallized (esl2 = 430 M-' cm-I). Triphenylphosphine (Aldrich) was twice recrystallized from ethanol-water. Triethylphosphine, diethylphenylphosphine, and diphenylethylphosphine (Strem) and N-methylmorpholine N-oxide hydrate (Eastman Kodak) were used as received. Dimethyl sulfoxide (Fisher) was distilled from CaH2, degassed, and stored under dinitrogen. 1,2-Dichloroethane (Baker) was distilled from P4010,degassed, and stored under dinitrogen or argon over 4-A molecular sieves. Kinetics Measuremts. Preparation of 1,2-dichloroethane solutions was performed anaerobically, and all measurements were made on solutions under an argon atmosphere. Reaction systems were monitored spectrophotometrically a t 512 nm. At this wavelength, a ,A, value of M O ~ O , ( S ~ C N Esubstantial ~~)~, absorbance changes occur over the course of reaction, in contrast to other wavelengths where there is appreciable a b ~ o r b a n c e .In ~ ~a typical experiment a cell containing a known concentration of M O O ~ ( S ~ C N(-2 E ~ ~mM) )~ was fitted with a serum cap and placed in a thermostated cell compartment at 25 OC. After thermal equilibration, a known amount of substrate in solution (5-100 mM, also at 25 "C) was injected through the serum cap and the cell contents were quickly mixed by shaking. The time dependence of absorbance was measured with a Cary Model 17D spectrophotometer. For each phosphine substrate two experiments were performed using different initial phosphine concentrations. The initial phase of the reaction was examined with a Durrum-Gibson Model 13701 stopped-flow apparatus, combined with a & i s Model PMQ I1 spectrophotometer and a Hewlett-Packard 6281A power supply. The photomultiplier detector was powered by a Hewlett-Packard Harrison Model 6515A dc power supply. Output from the photomultiplier was displayed as transmittance vs. time on a Tektronix type 564 storage oscilloscope, and traces were recorded photographically. The drive syringes and cell block were thermostated and ~ ~phosphine )~ salutions at 25 "C. Equal volumes of M O O ~ ( S ~ C N E of known concentrations were mixed in the cell, which had first been flushed with the phosphine solution. Kinetics of reverse reaction 2 in systems containing MoO(S,CNEt,), and Me2S0were determined by spectrophotometric monitoring of the time dependence of the absorbance at 512 nm. Procedures were similar to those used in the M ~ O ~ ( S ~ C N E t ~ ) ~ / p h o s psystems. hine The reaction with N methylmorpholine N-oxide proved too rapid for the measurement of kinetics by conventional spectrophotometry. All computations for data analysis were performed with locally written programs on a VAX 11-780 computer.
Results Oxygen Atom Transfer to Phosphine Substrates. With reference to generalized irreversibleMreaction 2 and reversible
(28) Tanaka, T.; Tanaka, K.; Matsuda, T.; Hashi, K. In ref 5, pp 361-367. (29) Nakamura, A.; Ueyama, N. In ref 5 , pp 369-377. (30) Nakamoto, M.; Shimizu, K.; Tanaka, K.: Tanaka, T. In0r.e. - Chim. Acra 1981, 53, L51. (31) Moore, F. W.; Larson, M. L. Inorg. Chem. 1967, 6, 998.
Reynolds, Berg, and Holm Table I. Kinetics and Equilibrium Parameters for Reactions 2 (X = PPh.) and 3 of Dithiocarbamate Comolexes L S,CNPr, S,CNEt,
solvent
t, "C
dataatb
ref
o-C,H,CI, C,H, MeCN
41 k , = 2.3, K, = 4 X lo-, 11 24 k , = 0.12, E,, = 14 17 25 k, = 1.1 t 0.3, AH,* = 8.4 I 18,19 0.5, AS,* =-30 I 1.6 CH,C1, 25 K , = 2.1 X lo-,, k , = 4.9, 22 k-, = 2350 1,2-C,H4CL, 25 AH, = -29.0 i 2.5 20 1,2-C,H4CL, 25 K , = (2.0 I0.2) X k , = 27, 28 2.93 I 0.29, k - , = 1470 I 20, Ea, = 17.6 I 0.4, AH,*= 17.0 I0.4, AS3* = 0.1 * 0.8, AH, = 12.1 t 0.6, AS, = 27.7 t 3.4
a Reactions denoted by subscripts. Units: k,, k.,, M-I scl ; k,, s-' ;Ea, AH,AH*, kcal/mol; AS, AS*, cal/(K mol); K , , M.
reaction 3, the rates of concentration change with time of each species are given by eq 4-1. With the assumption that the
-
d[MoO,Lzl dt [MoO2LzI(-kz[Xl - k-3[MoOL21)
k3[M02O&,] (4)
rate of dimerization is very fast compared to oxygen atom transfer (k3 >> k2),the system is always at equilibrium with respect to the dimerization process. The rate of this process depends on both the rate constant k-3 and the concentrations of M002L2and MoOL2. At very early times in the reaction system, the concentration of MoOL2 is quite small, affording a relatively slow rate of dimerization. At longer times, the amount of M0203L4 increases until depletion of M002L2by reaction with X drives reaction 3 toward dissociation. If Mo2O3L4is the most strongly absorbing species at a given wavelength, it is expected that the absorbance will increase as this species accumulates and decrease as it dissociates. The systems M o O ~ ( S ~ C N E ~ ~ ) ~ / P P (~n, -=, E0-3) ~ , in 1,2-dichloroethane solutions at 25 "C were subjected to a kinetics investigation. Coupled reactions 8 and 9, specific cases M002(S2CNEt2)2 + PPh,-,Et, MOO(SZCNEt2)2 + OPPh+,Et, (8) k,
+
Mo,O,(S,CNE~~)~ MoO(S2CNEt2), + Mo02(S2CNEt,),
(9)
of reactions 2 and 3, occur in these systems. Kinetics and equilibrium data from prior studies are collected in Table I. Reaction 8 has previously been examined with PPh3as the only phosphine substrate. The products of this reaction and the existence of equilibrium 9 have been abundantly demonstrated in earlier work. In addition, the distorted cis-octahedral structure of MoO2(SZCNR2)2(R = n-Pr,32Et33), the p o x 0 arrangement of M O ~ O ~ ( S ~ C (R N R=~n-Pr,32 ) ~ Et34),and the ~
(32) Ricard, L.; Estienne, J.; Karagiannidis, P.; Toledano, P.; Fischer, J.; Mitschler, A.; Weiss, R. J . Coord. Chem. 1974, 3, 277. (33) Berg, J. M.;Hodgson, K. 0. Inorg. Chem. 1980, 29, 2180.
Inorganic Chemistry, Vol. 23, No. 20, 1984 3059
Intermediate Oxo-Bridged Mo(V) Dimer Formation
0.0025 I
MOOJS2CNEt&
PPh, *
I
o = obierved
-
0.8
= calculated
MoO(S,CNEt,),
,
\ /
0.0 0
'
I 500
1000.
1500.
2000.
2500.
00
02
04
08
OB
10
3000. oxo -abstraction reaction coordinate
time (sec)
Figure 1. Plot of observed and calculated time dependence of absorbance at 512 nm for a system initially containing 2.16 mM Mo02(S2CNEt2)2 101 mM PPh,. The parameters of best fit, used in this and the following figures, are given in the text.
+
Figure 2. Plot of the concentrations of Mo(IV,V,VI) complexes with
time for a system initially containing 2.16 mM Mo02(S2CNEt2)2 and 101 mM PPh,, calculated from eq A-1, A-3, and A-4. The reaction coordinate refers to the fraction of 1 equiv of PPh3 consumed in reaction 4.
Table 11. Rate Constants for Oxygen Atom Transfer Reactions square-pyramidal configuration of MOD(S,CN-~-P~,),~~ have been established by X-ray diffraction. Consequently, the [ M o l , [substrate], Mo(IV,V,VI) species involved in reactions 8 and 9 are wellsubstrate mM mM rmaw,s kl: M - I s - l defined. PPh, 2.16 101 119 0.071 (3)a The reaction of M o O , ( S ~ C N E ~ ,with ) ~ a 25- or 50-fold 50.6 256 excess of PPh, gave an absorbance vs. time curve, shown in PPh,Et 2.27 10.1 379 0.23 (2) Figure 1, consistent with the foregoing prediction. The initial 5.05 918 PPhEt, 1.72 10.2 213 0.43 (2) yellow color of the reaction system, due to MoO,(S~CNE~,)~, 5.08 427 changed to violet (Mo,O~(S,CNE~,)~)and then to red PEt, 2.08 9.20 191 0.53 (1) (MoO(S,CNEt,),) as the reaction proceeded. The kinetics 2.37 10.5 173 of the system have been analyzed in detail, with the only Me,SO 2.01 1000 5019 0.00016 (l)b assumption being that the dimerization equilibrium is rapidly 2.12 1000 5020 established at all points along the reaction coordinate. This a Values calculated from eq A-17 with use of e4 = 430 M-' cm-' treatment, developed in the Appendix, permits calculation of and e6 = 18 M-I cm-'. For each run, two values of k, were calabsorbance vs. time (eq A-15), dependent only on the unknown culated with use of K = 0.0014 M, e5 = 25 000 M-' cm-l and K = parameters K,kl, and c5, the molar extinction coefficient of 0.0020 M, E $ = 30 500 M-' cm-I, which represent acceptable exM o ~ O ~ ( S , C N E ~The ~ ) ~values . of e5 and K are independent tremes of these parameters. The value quoted is the mean of four values so obtained. The esd values in parentheses were calculated of substrate, and kl for a particular substrate can be deterfrom the equation u = [E& (xi - K ) z / ( n - l ) ] k-i, reaction mined from K,e5, and t,,, the time at maximum absorbance 10. (qA-17). Absorbance data for the system M O O ~ ( S ~ C N E ~ ~ ) ~ /comparable PP~~ quality. The uncertainty in kl was calculated by were analyzed by finding the values of c5 and K, and the using limiting values of cs and K in eq A- 17 to give boundaries corresponding value of kl,which minimize the function f = on the rate constant. &wij(Aijdd where i j refers to the jth point in the For the other substrates PPh,-,Et, (n = 1-3), the rate ith experiment and wij = l/(ni(AirM)2). The best fit gave constants kl were calculated from the time at maximum abvalues of c5 = 28 000 f 2500 M- cm-', K = (1.7 f 0.3) X sorbance using the values of e5 and K obtained from the PPh, lo-, M, and kl = 0.071 f 0.003 M-' s-*. Values of the first system. Values of the rate constants for the complete set of two parameters are in good agreement with ~ ~ ( 5 1nm) 2 = phosphine substrates, listed in Table 11, are the average values 24000 M-' cm-' and K = (2.0 f 0.2) X lo-, M determined from two experiments. Uncertainties in these values were by Matsuda et al.,' in their study of equilibrium 9 in 1,2calculated as described above. dichloroethane solution by the concentration-jump method. In order to demonstrate the slow rate of dimerization (reThe satisfactory conformance between the observed and action 9) at short reaction times, the system Moo2calculated time dependence of absorbance is illustrated in (S,CNEt2),/PPh3 was examined for the first 100 ms by Figure 1. The time dependence of the concentrations of the stopped-flow spectrophotometry. The results are plotted in Mo(IV,V,VI) complexes is provided in Figure 2. At the Figure 3. The solid line represents a numerical solution of reaction coordinate 0.5, the time at which 0.5 equiv of PPh3 eq 4-7 using the above values of kl (=IC,) and e5, the k3 and is consumed in reaction 8, the mole fraction of Mo203k-, values of Matsuda et aL2' (Table I), and initial concen(S2CNEt2), is 0.18, its maximum value. At this point the mole trations of MoO2(S2CNEt,), and phosphine. The dotted line, fractions of M o O ~ ( S ~ C N E and ~ , ) ~MoO(S2CNEt2), are both calculated from eq A- 15 (converted to transmittance), de0.41. scribes the case where equilibrium is instantaneously estabIn this treatment, the parameters c5 and K are strongly lished at all points. It is identical with the curve obtained from correlated; Le., increasing or decreasing both by appropriate the numerical solution of eq 4-7 at the limit of infinite k3 and amounts gives fits that are essentially as satisfactory as that k-,. The experimental points do not fall off as rapidly with obtained with the above values. Meaningful standard deviatime as does the dotted line, indicating that equilibrium has tions for these parameters, therefore, cannot be calculated. For not been established during this initial period. this reason, the uncertainties given for 9 and K are estimates Oxygen Atom Transfer from Substrates. The reverse of obtained by finding values of the parameters that gave fits of reaction 2 has been examined in the form of reactions 10 and 11. The kinetics treatment is equivalent to that described for forward reaction 2. The rate constant is calculated from eq (34) Garner, C. D.; Howlader, N. C.; Mabbs, F. E.;McPhail, A. T.; Onan, A-17 with C = [Mo(IV)], P = [substrate], and kl = kl and K. D. J . Chem. SOC.,Dalton Tram. 1979, 962.
3060 Inorganic Chemistry, Vol. 23, No. 20, 1984 MoO(S&NEt,),
MoO(S,CNEt,),
8 0.8 e
2c" e
'1
i",','
,
0.4
1
D
+
+
Reynolds, Berg, and Holm
Me,SO
-lJ 4
OZN
